An available method for implementing extended depth of field (hereafter EDOF) is to convolute images uniformly focusing in the depth direction by moving a focal lens or an imaging element during an exposure time, and perform image restoration processing using a blur pattern which is obtained in advance by measurement or simulation, to thereby obtain an EDOF image (Non-patent Document 1).
A primary application example of the EDOF technology is for imaging with a microscope. In this application, the EDOF technology is known to be rational because the image restoration processing method after exposure, using a single blur pattern, can be applied if a way of moving a focus lens or an imaging element is controlled such that the blur of an image always becomes uniform (Patent Document 1).
In the application example however, focus must be controlled upon driving the focus lens or the imaging element, so that the imaging surface moves at a constant velocity (Non-patent Document 1).
Therefore the movement pattern is demanded to have a constant velocity from the rear side focusing end position to the front side focusing position, or in the opposite direction thereof.
Another application example of EDOF technology is downsizing a camera installed in a portable telephone or the like. In other words, using the EDOF effect, an all focused image (an image in which all objects are focused) can be obtained without including an auto focus mechanism.
Another application example of EDOF technology is an application to a standard digital still camera and digital video camera. As a recent trend in digital still cameras and digital video cameras, simpler image capturing with less error is demanded, and EDOF technology is expected as a technology that can implement an all focused image, which is image capturing without focusing error.
In order to apply the EDOF method to a digital still camera and a digital video camera like this, continuous image capturing without generating a delay between frames is demanded upon capturing moving images, therefore it is known that reciprocating movement as shown in FIG. 14 is performed when capturing a moving image, assigning one video frame respectively to the advance movement and return movement of a focus lens or an imaging element, whereby EDOF moving image capturing is enabled.
However a displacement pattern of a focus lens or a displacement pattern of an imaging element shown in FIG. 14 includes a return movement at an acute angle at a closest end or most distant end from the object. In order to implement this return movement at an acute angle, a large thrust must be generated momentarily in an actuator for driving the imaging element or the focus lens. In terms of downsizing and conserving power of an apparatus, the reciprocating movement control that generates such a large thrust is not practical for a portable digital still camera or digital video camera. Furthermore this kind of reciprocating movement control momentarily generates a large thrust, and suddenly inverts the velocity, hence the driving mechanism quickly wears out, and vibration and noise during driving are large, which is not acceptable in terms of quality.
Available conventional movement control apparatuses which reciprocate an optical element such as a focus lens or an imaging element in the optical axis direction, which may be possible to be used to implement EDOF for capturing a moving image or a still image of an object, are the movement control apparatuses disclosed in Patent Document 2 and Patent Document 3.
According to the technology disclosed in Patent Document 2, as illustrated in FIG. 15, a stator 113 constituted by a yoke 116 facing an outer surface of a cylindrical permanent magnet via a space, and a movable element 127 which has a driving coil 129 that can slide in the axis direction with respect to the stator 113, are disposed, an air-core coil 132, as a sensor coil, is disposed outside the yoke 116, and a permanent magnet 128, which displaces in the air-core 132 as the movable element 127 displaces, is installed in the movable element 127. Since the electromagnetic induction function to the air-core coil 132 by the driving coil 129 is magnetically shielded by the yoke 116 located therebetween, only an electromotive force in accordance with the displacement velocity of the permanent magnet 128 interlocking with the movable element 127, which is only a velocity signal, is generated in the air-core coil 132. The position of the movable element 127 is controlled by the position detection voltage of a position sensor 161, and by damping the movable element 127 using the velocity signal, which is the output of the air-core coil 132, response can be improved without generating hunching.
According to the technology disclosed in Patent Document 3, as illustrated in FIG. 16, a focus lens 110 is driven in the optical axis direction by an actuator that is constituted by a driving coil 135 and a magnet 134 and is disposed coaxially around the optical axis of the focus lens 110, and position control is performed using a position signal of a position sensor that is constituted by an inclined magnet 139, of which magnetic flux changes as the focus lens 110 moves, and a Hall element, and a velocity signal of a moving velocity detection coil 137 of the focus lens 110. Since the velocity detection coil 137 is wound around a bobbin 131 on which the driving coil 135 is wound, a magnet for a sensor can be used for driving as well, which allows decreasing a number of components, decreasing weight and decreasing cost.
In order to extend the depth of field, the optical element is moved at a constant velocity for the amount of a focal distance which corresponds to the depth of field to be extended. For this purpose, a moving pattern of the optical element is generated, and high-speed positioning control is performed on the optical element in accordance with the target position of the pattern.
In the case of the actuator having a conventional configuration, however, the positioning of the focus lens is controlled basically by feeding back the position signal outputted by the position sensor to the control circuit. Therefore the moving distance of the focus lens is long, and the focus lens must be moved at a constant velocity. As a result, the position detection range of the lens is long, that is, an entire operation range in the movable area, and a position sensor which excels in position detection accuracy and linearity is required. Furthermore, a velocity sensor to obtain a velocity signal, for damping the movable element upon positioning the actuator so that vibration is not generated, is required. This makes the apparatus large and expensive.    Patent Document 1: Japanese Patent Application Laid-Open No. H5-313068    Patent Document 2: Japanese Patent Application Laid-Open No. H1-206861    Patent Document 3: Japanese Patent Application Laid-Open No. H4-119306    Non-patent Document 1: H. Nagahara, S. Kuthirummal, C. Zhou and S. Nayar: “Flexible Depth of Field Photography”, European Conference on Computer Vision (ECCV), October 16th, Morning Session 2: Computational Photography (2008)